By Harold McGee
The natural separation of cream from milk by means of gravity takes 12 to 24 hours, and was superseded late in the 19th century by the merry-go-round forces of the French centrifugal separator. Once separated, the cream is pasteurized. In the United States, the minimum temperatures for pasteurizing cream are higher than the milk minimum (for 20% fat or less, 30 minutes at 155°F/68°C; otherwise at 165°F/74°C). “Ultrapasteurized” cream is heated for 2 seconds at 280°F/140°C (like UHT-treated milk; however the cream is not packaged under strictly sterile conditions, and so is kept refrigerated). Under refrigeration, ordinary pasteurized cream keeps for about 15 days before bacterial activity turns it bitter and rancid; ultrapasteurized cream, which has a stronger cooked flavor, keeps for several weeks. Normally cream is not homogenized because this makes it harder to whip, but long-keeping ultrapasteurized cream and relatively thin half-and-half are usually homogenized to prevent continuing slow separation in the carton.